The four Specific Aims of this proposal will advance our understanding of behavioral and neurological developmental processes. Specific Aim I will identify how olfactory experience during nursing determines adult sexual behavior. Specific Aim II explores the mechanisms that underlie weaning. Through judicious rearing we have been able to extend suckling through the time of sexual maturity. In Specific Aim III I will analyze the contribution of the endogenous opioid systems to different motivational and associative states in very young (Day 5) rats. We have already obtained large learned preferences for substances associated with very low morphine doses (0.5 mg/kg). Specific Aim IV is concerned with central development. It will study the ontogeny of negative affect through electrical stimulation of the ventromedial hypothalamus. We have already discovered that such stimulation elicits unmistakably aversive behavior in Day 3 rats. We will also identify by 2-DG autoradiography neural tissue that is differently activated during the MFB stimulation that elicits complex motor behaviors. This approach will also be used in an effort to identify central tissue involved in olfactory conditioning in Day 3 rats.